1. Field of the Invention
The present invention relates to a process for the preparation of molecular sieve silicas, particularly calcined silicas. In particular the present invention relates to the use of water soluble silicates and non-ionic polyoxyethylene oxide PEO based surfactants for the preparation of the silicas which have thermal stability.
2. Description of Related Art
Mesoporous silicas are useful in a variety of applications. Calcined silicas are useful in refining if they are stable at temperatures between 600xc2x0 C. to 800xc2x0 C.
Mesoporous molecular sieve silicas with wormhole framework structures (e.g., MSU-X (Bagshaw, S. A., et al., Science 269 1242 (1995); Bagshaw, S. A., et al., Angwen. Chem., Int. Ed. Engl., 35 1102 (1996); and Prouzet, E., et al., Angwen. Chem., Int. Ed. Engl., 36 516 (1997)), and HMS (Tanev, P. T., et al., Science 267 865 (1995)) are generally more active heterogeneous catalysts in comparison to their ordered hexagonal analogs (e.g., MCM-41 (Beck, J. S., et al., J. Am. Chem. Soc., 114 10834 (1992)), SBA-3 (Huo, Q., et al., Nature 368 317 (1994)), and SBA-15 (Zhao, D., et al., J. Am. Chem. Soc., 120 6024 (1998)). The enhanced reactivity has been attributed, in part, to a pore network that is connected in three dimensions, allowing the guest molecules to more readily access reactive centers that have been designed into the framework surfaces (Zhang, W., et al., Stud. Surf. Sci. Catal., 117 23 (1998); Reddy, J. S., et al., J. Chem. Soc., Chem. Commun., 1059 (1994); Reddy, J. S., et al., J. Chem. Soc., Chem. Commun. 2231 (1995); Sayari, A., Chem. Mater. 8 1840 (1996); Mokaya, R., et al., J. Catal., 172 211 (1997); and Kloetstra, K. R., et al., J. Chem. Soc., Chem. Commun., 228 (1997)). All of the wormhole framework structures reported to date have been prepared through supramolecular Sxc2x0Ixc2x0 (Tanev, P. T., et al., Science 267 865 (1995)) and Nxc2x0Ixc2x0 (Bagshaw, S. a., et al., Science 269 1242 (1995); Bagshaw, S. A., et al., Angwen. Chem. Int. Ed. Engl., 35 1102 (1996); and Prouzet, E., et al., Angwen. Chem. Int. Ed. Engl., 36 516 (1997)) assembly pathways where Ixc2x0 is an electrically neutral silica precursor (typically, tetraethylorthosilicate, TEOS), Sxc2x0 is a neutral amine surfactant, and Nxc2x0 is a neutral di- or tri-block surfactant containing polar polyethylene oxide (PEO) segments. One disadvantage of these pathways, as with other assembly pathways based on TEOS, is the high cost of the hydrolyzable silicon alkoxide precursor. Greater use of wormhole framework structures as heterogeneous catalysts may be anticipated if a more efficient approach to either Sxc2x0Ixc2x0 or Nxc2x0Ixc2x0 assembly could be devised based on the use of low cost soluble silicate precursors, without sacrificing the intrinsically desirable processing advantages of these pathways (e.g., facile removal and recycling of the surfactant). Related patents are: U.S. Pat. Nos. 5,622,684, 5,795,559, 5,800,799 and 6,027,706 to Pinnavaia et al.
Recently, Guth and co-workers reported the preparation of disordered silica mesostructures by precipitation from sodium silicate solutions over a broad range of pH in the presence of Triton-X 100, a Nxc2x0 surfactant (Sierra, L., et al., Adv. Mater., 11(4) 307 (1999)). The retention of a mesostructure was observed up to a calcination temperature of 480xc2x0 C., but the complete removal of the surfactant at 600xc2x0 C. led either to the extensive restructuring of the silica framework, as indicated by the loss of mesoporosity or the formation of a completely amorphous material.
There is a need for mesostructures structurally stable to calcination temperatures in excess of 600xc2x0 to 800xc2x0 C.